Tangential Quadrilateral

In geometry, a tangential quadrilateral is a convex quadrilateral whose sides are all tangent to a single circle within the quadrilateral. Sometimes it is called a tangent quadrilateral, circumscribed quadrilateral, or circle inscribed in a quadrilateral. The circle is called the incircle of the quadrilateral or its inscribed circle. The center of the circle is called the incenter and its radius is called the inradius.

According to Pitot's theorem, the two sums of the lengths of opposite sides are equal.

A circle can be inscribed in a quadrilateral if and only if the angle bisectors of the four angles of the quadrilateral are concurrent.